Sera from 3 of 30 adults vaccinated withHaemophilus influenzaetype b polysaccharide vaccine (Hib PS) had poor complement-mediated bactericidal activity despite the presence of anti-Hib PS antibody concentrations of 8.6 to 20.5 µg/niL. These “nonkiller” antibodies killed <0.4 log cfu/mL compared to >3 logs with all but one of the other sera. To investigate the basis of this poor functional activity, we characterized in detail the IgG antibodies to Hib PS present in two of the nonkiller sera, and compared the results with two of the “killer” sera. The latter were selected based on comparable levels of total antibody to Hib PS. No consistent differences were found between the relative proportions of IgG or IgA antibody to total anti-Hib PS antibody, or the respective ratios of IgG1 to IgG2 antibody in the nonkiller and killer sera. IgG fractions, and IgG affinity purified antibody to Hib PS were prepared. When tested at 2 µg/mL of antibody, the IgG fractions from the two nonkiller sera had much lower bactericidal activity than the corresponding fractions from the killer sera (3 logs less killing), and the former also had lower complement-mediated opsonic activity (20 and 13% uptake by human PMN compared to 62 and 93%). These data show the striking variability in the functional activity of vaccine-induced antibody to Hib PS. Antibody functional activity is likely to be affected by a number of factors but one important variable appears to be avidity since the IgG anti-Hib PS antibody from the two nonkiller sera had 2- to 5-fold lower avidity than the IgG antibody from the two killer sera. Therefore, measurement of antibody avidity in studies of Hib vaccine immunogenicity would be informative